Modern wind turbines are commonly used to supply electricity into the electrical grid. Wind turbines of this kind generally comprise a rotor with a rotor hub and a plurality of blades. The rotor is set into rotation under the influence of the wind on the blades. The rotation of the rotor shaft drives the generator rotor either directly (“directly driven”) or through the use of a gearbox.
A variable speed wind turbine may typically be controlled by varying the generator torque and the pitch angle of the blades. As a result, aerodynamic torque, rotor speed and electrical power generated will vary.
A common prior art control strategy of a variable speed wind turbine is described with reference to FIG. 1. In FIG. 1, the operation of a typical variable speed wind turbine is illustrated in terms of the pitch angle (β), the electrical power generated (P), the generator torque (M) and the rotational velocity of the rotor (ω), as a function of the wind speed.
In a first operational range, from the cut-in wind speed to a first wind speed (e.g. approximately 5 or 6 m/s), the rotor may be controlled to rotate at a substantially constant speed that is just high enough to be able to accurately control it. The cut-in wind speed may be e.g. approximately 3 m/s.
In a second operational range, from the first wind speed (e.g. approximately 5 or 6 m/s) to a second wind speed (e.g. approximately 8.5 m/s), the objective is generally to maximize power output while maintaining the pitch angle of the blades so as to capture maximum energy. In general, in the second operational range, the pitch angle of the blades may be substantially constant, although the optimal blade setting may theoretically depend on the instantaneous wind speed. In order to achieve this objective, the generator torque and rotor speed may be varied so as to keep the tip speed ratio λ (tangential velocity of the tip of the rotor blades divided by the prevailing wind speed) constant so as to maximize the power coefficient Cp.
In order to maximize power output and keep Cp constant at its maximum value, the rotor torque may be set in accordance with the following equation:T=k·ω2 , whereink is a constant, and ω is the rotational speed of the generator. In a direct drive wind turbine, the generator speed substantially equals the rotor speed. In a wind turbine comprising a gearbox, normally, a substantially constant ratio exists between the rotor speed and the generator speed.
In a third operational range, which starts at reaching nominal rotor rotational speed and extends until reaching nominal power, the rotor speed may be kept constant, and the generator torque may be varied to such effect. In terms of wind speeds, this third operational range extends substantially from the second wind speed to the nominal wind speed e.g. from approximately 8.5 m/s to approximately 11 m/s.
In a fourth operational range, which may extend from the nominal wind speed to the cut-out wind speed (for example from approximately 11 m/s to 25 m/s), the blades may be rotated (“pitched”) to maintain the aerodynamic torque delivered by the rotor substantially constant. In practice, the pitch may be actuated such as to maintain the rotor speed substantially constant. At the cut-out wind speed, the wind turbine's operation is interrupted.
In the first, second and third operational ranges, i.e. at wind speeds below the nominal wind speed (the sub-nominal zone of operation), the blades are normally kept in a constant pitch position, namely the “below rated pitch position”. Said default pitch position may generally be close to a 0° pitch angle. The exact pitch angle in “below rated” conditions however depends on the complete design of the wind turbine.
The before described operation may be translated into a so-called power curve, such as the one shown in FIG. 1. Such a power curve may reflect the optimum operation of the wind turbine under steady-state conditions.
In the supra-nominal zone of operation (wind speeds at or above the nominal wind speed), the maximum available energy in the wind stream is very consciously not captured. That is, the blades are actively pitched to a position in which they “catch” less wind, and generate less torque than possible. This is mainly done to limit the loads on the wind turbine.
The point on the power curve representing the electrical power generated at nominal wind speed is generally referred to as the “power curve knee”.
The power curve may be regarded as the key performance characteristic establishing the power output that may be expected from a wind turbine. The power curve is thus also often used to predict a wind turbine's profitability. Based on the wind data available for a specific site, and on the power curve, an expected energy output over e.g. a month, a year or a number of years may be predicted.
The wind data that may be available for a specific site is gathered over a period of time and is generally determined based on 10 minute averages. That is, average wind speed(s) and direction(s) as calculated over 10 minutes of time are the outcome of this process and form the input for a possible calculation of expected energy output of wind turbine. Also, in the evaluation of the performance of a wind turbine or a wind park such 10 minute averages may be used. Obviously other time intervals could also be used.
It has been found that wind turbines and wind parks regularly underperform with respect to the given power curve. Especially, in and around the “power curve knee”, the generated power at average wind speeds equal to or slightly different than the nominal wind speed, the electrical output of a wind turbine is generally less than expected. This is mainly because if an average wind speed over 10 minutes is determined to correspond to the nominal wind speed, this inevitably means that for some time the instantaneous wind speed was below the nominal wind speed, and for some time the instantaneous wind speed was actually above the nominal wind speed. Because of the operation strategy implemented in wind turbines, the output power at instantaneous wind speeds above the nominal wind speed is “capped” at the nominal power (the generated electrical power does not increase at higher wind speeds), whereas at instantaneous wind speeds below the nominal wind speed, the generated electrical power does decrease.
The power curve is thus rounded or flattened around the “power curve knee”. The extent to which the electrical power output is reduced compared to the expected value is dependent mainly on wind variability, i.e. turbulence. Over the life time of a wind turbine, the wind speed may relatively often be close to the nominal wind speed. The problem of rounding of the power curve knee thus should not be underestimated.
The present disclosure relates to various methods and systems for avoiding or at least partly reducing this problem.